Transcript Newton*s Laws

Newton’s Laws
Aim: How do Newton’s Laws affect moving
objects?
Newton’s First Law
An object in motion (or at rest) will stay in motion
(or at rest) unless acted on by an unbalanced force
“Law of Inertia”
Inertia – The resistance of an object to change in
its motion; directly proportional to the objects
MASS
Which has more Inertia?
100m/s
Inertia = Mass; Speed has NO affect on Inertia!
Newton’s Third Law
For every action there is an equal and opposite
reaction
Action
Reaction
If the sumo wrestler slams into the kid with 100
Newtons of force, how much force does the kid hit the
sumo wrestler?
100N
100N
For EVERY action there is an EQUAL and
OPPOSITE reaction
Newton’s Second Law
The acceleration of an object is directly
proportional to the force exerted on it but
inversely proportional to the object’s mass
Fnet  ma
Acceleration
Units : m 2
s
Force
Mass
 kg  m 
Units : Newtons (N)  2 
 s 
Units: Kilograms (kg)
Sir Isaac Newton
• Dec. 25th 1642 – March
20th 1726
• Invented Calculus & the
Reflecting Telescope
• Believed to have died a
virgin due to being
deeply religious and
wasn’t “subject to
frailties of man”
Example #1
How much force is required to accelerate a
20kg crate at 2m/s2?
Fnet  ma  Fnet  (20kg )(2 m s 2 )  Fnet  40 N
Example #2
A 1200kg car slows to a stop in 3 seconds when
the brakes apply a force of 2.0×104N. How fast
was the car going?
Fnet  ma   20,000 N  (1200kg )a 
a  16.67 m s 2
v f  vi  at  0 m s  vi  (16.67 m s 2 )(3s) 
vi  50 m s
Example #3
The combined mass of a child on a bike is 75kg.
If the child rides at a constant velocity, what
net force acts on the child?
0N
If something is either “STATIONARY” -OR“MOVING AT A CONSTANT SPEED/VELOCITY”
Velocity does
NOT CHANGE
Acceleration
= 0m/s2
Net Force = 0N
EQUILIBRIUM
Weight
The amount of force gravity pulls on an object
ALWAYS STRAIGHT DOWN!
Fg  mg
Gravity
9.81 m 2
s
Weight
Mass
 kg  m 
Units : Newtons (N)  2 
 s 
Units: Kilograms (kg)
WEIGHT ≠ MASS
Mass and Weight are NOT THE SAME
• Mass is measured in KILOGRAMS (kg)
• Weight is measured in NEWTONS (N)
However, they are RELATED
• More mass  More weight
• Stronger gravity  More weight
Generally, MASS remains CONSTANT
SUMMARY
• Describe Newton’s 1st Law of Motion
• Describe Newton’s 2nd Law of Motion
• Describe Newton’s 3rd Law of Motion
• How do you calculate force?
• What are the units for force?